This invention relates to a regenerative fuel cell system. More particularly, this invention relates to a regenerative fuel cell apparatus which combines a fuel cell unit and an electrolyzer unit, and method of use thereof.
Fuel cells have been proposed as a clean, efficient and environmentally friendly power source that has various applications. A conventional proton exchange membrane (PEM) fuel cell is typically comprised of an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. A fuel cell generates electricity by bringing a fuel gas (typically hydrogen) and an oxidant gas (typically oxygen) respectively to the anode and the cathode. In reaction, a fuel such as hydrogen is oxidized at the anode to form cations (protons) and electrons by the reaction H2=2H++2exe2x88x92. The proton exchange membrane facilitates the migration of protons from the anode to the cathode while preventing the electrons from passing through the membrane. As a result, the electrons are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts with electrons returned from the electrical circuit to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction by-product following xc2xdO2+2H++2exe2x88x92=H2O. On the other hand, an electrolyzer uses electricity to electrolyze water to generate oxygen from its anode and hydrogen from its cathode. Similar to a fuel cell, a typical solid polymer water electrolyzer (SPWE) or proton exchange membrane (PEM) electrolyzer is also comprised of an anode, a cathode and a proton exchange membrane disposed between the two electrodes. Water is introduced to, for example, the anode of the electrolyzer which is connected to the positive pole of a suitable direct current voltage. Oxygen is produced at the anode by the reaction H2O=xc2xdO2+2H++2exe2x88x92. The protons then migrate from the anode to the cathode through the membrane. On the cathode which is connected to the negative pole of the direct current voltage, the protons conducted through the membrane are reduced to hydrogen following 2H++2exe2x88x92=H2.
It is well known in the art that one type of regenerative fuel cell system combines separated fuel cell and electrolyzer units so that during the fuel cell mode of the system, the fuel cell unit generates electricity while consuming fuel gas (typically hydrogen) and oxidant (typically oxygen or air) and during the electrolyzer mode of the system, the electrolyzer unit generates the two process gases for consumption by the fuel cell unit, i.e. oxygen and hydrogen, while consuming electricity. Individual fuel cell and electrolyzer cells are usually interconnected in a series arrangement, often called xe2x80x9cstacksxe2x80x9d.
U.S. Pat. No. 5,376,470 entitled xe2x80x9cRegenerative Fuel Cell Systemxe2x80x9d and No. 5,506,066 entitled xe2x80x9cUltra-Passive Variable Pressure Regenerative Fuel Cell Systemxe2x80x9d, both issued to Rockwell International Corporation, disclose such a regenerative fuel cell system. The regenerative fuel cell system comprises a fuel cell including an anode for receiving hydrogen and a cathode for receiving oxygen, an electrolyzer for electrolyzing water to produce pure hydrogen and pure oxygen, storage tanks to respectively store hydrogen and oxygen from the electrolyzer, a water storage tank communicating with the fuel cell and the electrolyzer. The fuel cell is located above the water storage tank while the electrolyzer is located below the water storage tank. Hydrogen is supplied to the fuel cell during fuel cell mode or extracted from the cathode side of the electrolyzer during electrolyzer mode via a hydrogen line that is connected to the hydrogen storage tank and through a liquid-gas separator. Similarly, oxygen is supplied to the fuel cell via lines and through the water storage tank during fuel cell mode or extracted from the anode side of the electrolyzer via an oxygen line and through the water storage tank. The oxygen, when reaching the water storage tank, bubbles up to the fuel cell via a supply line during fuel cell mode or to the oxygen storage tank, when in the electrolyzer mode.
However, these regenerative fuel cell systems cannot meet the increasingly demanding requirement for fuel cell stacks. The systems are usually large in size and heavy in weight and require complex plumbing and ancillary equipment such as valves and controls. As is known in the art, the performance of the fuel cell unit in this system cannot be optimized unless an additional humidification device is provided to humidify the process gases and an additional heat exchanger is included to facilitate the heat dissipation, all of which results in increased system size and weight. When switching from electrolyzer mode to fuel cell mode, the fuel cell unit in the conventional regenerative fuel cell systems is cold and therefore is unable to achieve full power output until the stack is warm.
Moreover, at present there is an expanding interest in vehicular applications of fuel cell stacks, e.g. as the basic power source for cars, buses and even larger vehicles. Vehicular applications are quite different from many stationary applications. In stationary applications, fuel cell stacks are usually used as an electrical power source and are simply expected to run at a relatively constant power level for an extended period of time. In contrast, in a vehicular, particularly an automotive environment, the actual power required from the fuel cell stack can vary significantly. Moreover, the fuel cell stack is expected to respond rapidly to changes in power demand while maintaining high efficiencies. Further, for vehicular, particularly automotive applications, a fuel cell power unit is expected to operate under a disparate range of ambient temperature and humidity conditions. In addition, during regenerative braking period, the prior regenerative fuel cell systems are unable to capture the electricity to recharge the system due to their slow switchover times, making them less efficient. All these requirements are exceedingly demanding and make it difficult to incorporate a conventional regenerative fuel cell system into a vehicle and operate efficiently.
In view of the disadvantages and drawbacks associated with conventional regenerative fuel cell systems, it is desirable to provide a regenerative fuel cell system that enables improved fuel cell performance, including rapid switchover between fuel cell and electrolyzer modes, instantaneous full power operation, higher power density, less peripherals and hence higher system efficiency.
According to a first aspect of the present invention, a regenerative fuel cell system is provided, comprising an electrolyzer portion and a fuel cell portion;
the electrolyzer portion has a closeable hydrogen inlet and a hydrogen outlet in communication with the cathode of the electrolyzer portion for conducting hydrogen, a gas bypass having a gas bypass inlet and a gas bypass outlet for conducting oxidant gas for fuel cell reaction to the fuel cell portion, a water inlet and an oxygen-water outlet for exhausting oxygen generated in electrolyzer operation and coolant water from the fuel cell portion out of the electrolyzer portion;
the fuel cell portion has a hydrogen inlet, a first closeable hydrogen outlet for exhausting excess hydrogen in fuel cell mode, a second closeable hydrogen outlet for exhausting hydrogen generated in the electrolyzer portion in electrolyzer mode, an oxidant gas inlet, an oxidant gas outlet, a coolant water inlet and a coolant water outlet; and
the hydrogen inlet of the fuel cell portion being in communication with the hydrogen outlet of the electrolyzer portion; the oxidant gas inlet of the fuel cell portion being in communication with the gas bypass outlet of the electrolyzer portion; and the water inlet of the electrolyzer portion being in communication with the coolant water outlet of the fuel cell portion.
Preferably, the fuel cell portion and the electrolyzer portion are in juxtaposition. More preferably, the fuel cell portion and the electrolyzer portion are stacked one on top of the other, with the electrolyzer portion on the top.
The fuel cell portion comprises at least one proton exchange membrane fuel cell having an anode bipolar plate and a cathode bipolar plate; and the electrolyzer portion comprises at least one proton exchange membrane electrolyzer cell having an anode bipolar plate and a cathode bipolar plate.
Preferably, the regenerative fuel cell system further includes a separator plate sandwiched between the fuel cell portion and the electrolyzer portion, said separator plate is provided with a hydrogen port functioning as the hydrogen inlet for the fuel cell portion and the hydrogen outlet for the electrolyzer portion, an oxidant gas port functioning as the oxidant gas inlet for the fuel cell portion and the gas bypass outlet for the electrolyzer portion, and a water port functioning as the coolant water outlet for the fuel cell portion and the water inlet for the electrolyzer portion.
More preferably, the said second closeable hydrogen outlet is in alignment with the said hydrogen inlet of the fuel cell portion. More preferably, a common current collector plate is sandwiched between the anode of the fuel cell portion and the cathode of the electrolyzer portion, and said current collector plate is provided with a hydrogen port functioning as the hydrogen inlet for the fuel cell portion and the hydrogen outlet for the electrolyzer portion, an oxidant gas port functioning as the oxidant gas inlet for the fuel cell portion and the gas bypass outlet for the electrolyzer portion, and a water port functioning as the coolant water outlet for the fuel cell portion and the water inlet for the electrolyzer portion. Alternatively, the common current collector plate is grounded.
More preferably, the said gas bypass of the electrolyzer portion is provided on the face of the anode bipolar plate of each electrolyzer cell facing away from the proton exchange membrane.
More preferably, the said separator plate further includes a switch means in fluid communication with the hydrogen outlet of the electrolyzer portion, the hydrogen inlet of the fuel cell portion and an external hydrogen storage means, and said switch means operatively switches between a first position in which it fluidly communicates the hydrogen outlet of the electrolyzer portion to the external hydrogen storage means, and a second position in which it fluidly communicates the hydrogen outlet of the electrolyzer portion with the hydrogen inlet of the fuel cell portion.
More preferably, the electrolyzer portion further includes an additional hydrogen outlet for supplying the hydrogen generated in electrolyzer mode to an external storage means, and a purge means that purges the water carried by the hydrogen generated in the electrolyzer mode.
According to a second aspect of the present invention, a method of operating the regenerative fuel cell system in the first aspect of the present invention is provided, wherein comprising:
in fuel cell mode, introducing hydrogen into the electrolyzer portion via the said closeable hydrogen inlet so that hydrogen flows across the cathode of the electrolyzer portion before entering the fuel cell portion for reaction; introducing oxidant gas into the electrolyzer portion via the said gas bypass inlet so that the oxidant gas flows along the gas bypass and leaves the electrolyzer portion before entering the fuel cell portion; introducing coolant water into the electrolyzer portion after the coolant water flows through the fuel cell portion so that the coolant water flows across the anode of the electrolyzer portion; and
in electrolyzer mode, closing the said closeable hydrogen inlet and running the hydrogen generated into the fuel cell portion through the fuel cell portion; introducing coolant water of the fuel cell portion into the anode of the electrolyzer portion after the coolant water flows through the fuel cell portion.
The structure of the regenerative fuel cell system according to the present invention provides significant advantages over the existing system. First of all, the switchover time between the two operation modes is reduced to a minimum because the exchange of water and gas streams between the electrolyzer and the fuel cell portions ensures the reactant gases and liquid are on the proper electrodes for each reaction. In addition, both the fuel cell portion and electrolyzer portion are able to achieve full power instantaneously after the system is switched from one mode to the other due to the exchange of water and gas streams between the two sections. This water and gas exchange maintains both the fuel cell and electrolyzer portions of the stack at full operating temperature as well as maintaining the fuel cell portion in a humidified condition. In fact, the electrolyzer portion functions as a humidification section for the fuel gases, i.e. hydrogen for the fuel cell portion so that the higher temperature of operation is possible without drying out the MEA of the fuel cells. The electrolyzer portion also functions as a heat exchanger for the fuel cell portion to dissipate heat as a result of the exchange of water and gases between the two portions. Further, the electrolyzer portion preheats the fuel cell supply gases, preventing condensation and flooding in the first cells of the fuel cell portion, a common problem with cold gas streams. This heat exchange process serves to warm up the electrolyzer portion itself, improving the performance of the electrolyzer when switching to the electrolyzer mode of operation. In addition, the present system even allows for simultaneous fuel cell and electrolyzer operation, eliminating switchover time between the two modes of operation. This further demonstrates the instant on capability of the regenerative fuel cell system, making it suitable for deployment in vehicular applications for the reasons outlined in the aforementioned background technology. The rapid switchover time enables the system to capture the electricity energy to recharge the system during regenerative braking period when it is applied in vehicular applications, thereby making the regenerative fuel cell system more efficient. This rapid switchover time also makes the system well suited to UPS power type applications where seamless transfer between power generation and power storage modes of operation are required.
Further, since the electrolyzer and fuel cell portions share the single water cooling and humidification loop, and since most of the cooling and humidification happens internally, the system requires less plumbing and less pipe or conduit components. Therefore the structure of the system is simplified, resulting in reduced size and weight.